CVE-2026-107720: fast-jwt signature verification bypass with null/empty key

CVE-2026-107720 · Severity: high · CVSS 7.4 · Published 2026-10-08

fast-jwt is a Node.js library for signing and verifying JSON Web Tokens (JWTs), which are used to securely transmit user identity and claims. The createVerifier function in versions 6.3.0 and earlier skips signature verification entirely when the signing key is an empty string or null and an algorithms allowlist is explicitly provided, allowing an attacker to forge JWTs with arbitrary claims without any signing key. This breaks JWT-based authentication and authorization, potentially allowing attackers to impersonate users or escalate privileges.

Technologies: Nearform Fast-Jwt, fast-jwt (npm). Vendors: Nearform, npm.

Quick answers

Is CVE-2026-107720 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-107720 being exploited in the wild.
How severe is CVE-2026-107720?
High severity, with a CVSS score of 7.4, published 8 October 2026.
Which products and versions are affected?
Nearform Fast-Jwt ≤ 6.3.0; fast-jwt (npm) <= 6.3.0, SEMVER:introduced=0,fixed=6.3.1.
Is there a fix for CVE-2026-107720?
Yes. fast-jwt (npm): upgrade to 6.3.1 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-107720 attacked?
It is attacked over the network.
Am I affected by CVE-2026-107720?
Check whether you run Nearform Fast-Jwt or fast-jwt (npm) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • fast-jwt (npm): upgrade to 6.3.1 or later
    npm install fast-jwt@6.3.1

Technical details

The vulnerability stems from four cooperating code paths: (1) falsy sync keys ('', null) bypass the prepareKeyOrSecret validation that rejects empty secrets; (2) explicit algorithms retain an allowlist even when key is falsy; (3) the signature presence check gates on hasKey being truthy, allowing unsigned tokens to pass validation; and (4) the signature cryptographic check itself is gated on signature being truthy. An attacker can present an unsigned JWT (header.payload.) to any application using createVerifier with key: '' or key: null plus an explicit algorithms allowlist, bypassing signature verification entirely while claim checks still execute.

Affected products

  • nearform fast-jwt ≤ 6.3.0

Identifiers

  • CVE-2026-107720
  • GHSA-8wpc-h4q6-8fxv

Timeline

  • 2026-10-08: disclosed

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